Dr. Nora Volkow, head of the National Institute on Drug Abuse, uses the brain scans of addicts to show how difficult it is to just say "no" to drugs and other addictive behaviors, including eating. The pioneering drug addiction researcher speaks to Morley Safer for a "60 Minutes" report to be broadcast on Sunday, April 29 at 7 p.m. ET/PT.

Scans of addicts' brains help to explain the chemistry of drug addiction, which is a disease that changes the brain, says Volkow. "We know that drug addiction is a chronic disease...drugs change the brain, physically change it," she says. The changes last long after the addict stops taking the drug, says Volkow.

Among her key findings is that levels of dopamine, the chemical that regulates the pleasure centers of the brain, rise if an addict should even see a photo of the substance they abused. She demonstrates the same is true if a hungry person sees food, showing Safer a brain scan of a hungry person being shown a picture of a hamburger.

The scan reveals a dopamine rush. "It just basically stimulates release of dopamine and the more they release, the more they want the food," Volkow tells Safer. "We always say, 'Well, why do we have a problem with obesity in our society?' And I said, 'My God, we are surrounded by stimuli with which we are conditioned. If you like hamburgers, you may see that McDonald's yellow arches and then dopamine goes inside your brain and you want [a hamburger] and you don't know why you want it," she tells Safer.

This may explain why there are so many obese people who cannot just say no to food, says Volkow. "If it were so easy [to just say no] I think that we would have no problem with obesity, we would have no problem with drugs." She points to her own difficulties in resisting the things she likes, such as chocolate and coffee, that she sometimes can't resist. "I'm a very wired person, so I shouldn't drink more coffee, but at some times, I can't resist that," she tells Safer. "So saying to someone, 'Just say no,' is magical thinking.